The resonant ejection scan is a well-known technique for performing mass analysis in an ion trap mass spectrometer. Generally described, the resonance ejection scan utilizes a supplemental oscillatory voltage applied across opposing electrodes of the ion trap. As the main trapping voltage is ramped, ions are brought into resonance in order of their mass-to-charge ratios. The amplitude of motion of the resonantly excited ions increases in the dimension defined by the opposing electrodes until the ions either strike the electrode surfaces or are ejected from the trap through one or more apertures aligned with the dimension of excitation. In a three-dimensional quadrupolar ion trap, resonantly excited ions are ejected from the trap in approximately equal numbers through two opposing apertures located in the end cap electrodes. However, because only those ions that exit the trap through one of the apertures are detected (the other aperture is employed for ion injection) about fifty percent of the ejected ions are lost, thereby adversely affecting sensitivity.
In a conventional two-dimensional (linear) quadrupolar ion trap, substantially all the ejected ions may be detected by adapting both opposed electrodes to which the resonance excitation voltage is applied (e.g., both central X rods) with elongated apertures or slots through which the resonantly excited ions may be ejected, and by providing two separate dynode/detector arrangements, each dynode/detector arrangement being positioned to detect ions ejected through one of the opposed slots. However, the inclusion of two separate dynode/detector arrangements can significantly increase the instrument complexity and manufacturing cost, particularly since each dynode/detector arrangement and its associated components typically require a dedicated power supply of significant expense. Of course, the cost of the instrument may be reduced by eliminating one of the dynode/detector arrangements and detecting only those ions that are ejected through one of the slots, but this configuration results in the loss of about half of the detectable ions and consequently produces a reduction in overall sensitivity of about 50 percent.
In view of the limitations of prior art ion trap mass spectrometers discussed above, there is a need for an ion trap mass spectrometer that avoids the high costs associated with multiple detectors, but which provides a substantially higher degree of sensitivity relative to known instrument designs in which a significant portion of the ejected ions are discarded.